1. Field of the Invention
The present invention relates to a pn junction type compound semiconductor light-emitting device. Specifically, the present invention relates to a pn-junction type compound semiconductor light-emitting device for multi-color light emission, where a light-emitting layer emitting multi-wavelength light is constructed by superposing a plurality of constituent layers.
2. Description of the Related Art
Group III nitride semiconductors such as gallium indium nitride (GaXIn1xe2x88x92XN: 0xe2x89xa6Xxe2x89xa61) have been heretofore used as a constituent material of a light-emitting layer for emitting short wavelength light such as blue light in a light-emitting diode (LED) (see, JP-B-55-3834 (the term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese patent publicationxe2x80x9d)). The band gap of the GaXIn1xe2x88x92XN (0xe2x89xa6Xxe2x89xa61) is known to non-linearly and abruptly change in correspondence with the gallium (Ga) composition ratio (=X) or indium composition ratio (=1xe2x88x92X) (see, JP-B-55-3834 supra). For example, in a hexagonal wurtzite crystal-structure GaXIn1xe2x88x92XN, the band gap at room temperature decreases from about 3.4 electron volt (unit: eV) of gallium nitride (GaN) to about 2.9 eV when the indium composition ratio is adjusted to 0.2 (see, JP-B-55-3834 supra). As such, the GaXIn1xe2x88x92XN (0xe2x89xa6Xxe2x89xa61) is advantageous in that the light emission wavelength can be changed by slightly changing the indium composition ratio (=1xe2x88x92X).
As for conventional multi-color light-emitting devices, a technique where a light-emitting layer giving multi-wavelength light is constructed from a plurality of gallium indium nitride (GaXIn1xe2x88x92XN: 0xe2x89xa6Xxe2x89xa61) layers having different indium compositions (=1xe2x88x92X) is disclosed. For example, JP-A-2001-168384 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) (published on Jun. 22, 2001) discloses a technique where a light-emitting layer giving multi-wavelength light is constructed from three GaXIn1xe2x88x92XN (0xe2x89xa6Xxe2x89xa61) well layers different from each other in indium (In) composition. Furthermore, for example, in JP-A-11-289108 (published on Oct. 19, 1999), an LED emitting two-wavelength light is constructed by superposing two GaXIn1xe2x88x92XN (0xe2x89xa6Xxe2x89xa61) layers having different indium compositions. A multi-wavelength LED having a light-emitting layer constructed by superposing a plurality of GaXIn1xe2x88x92XN (0xe2x89xa6Xxe2x89xa61) layers having different indium compositions is also described in JP-A-10-22525 (published on Jan. 23, 1998).
In conventional multi-color light-emitting devices having a light-emitting layer emitting multi-wavelength light with different light emission wavelengths, a pn-junction type hetero-junction structure light-emitting part is constructed. Particularly, for increasing the light emission intensity, the light-emitting part is constructed to have a double hetero (DH) structure (see, Iwao Teramoto, Handotai Device Gairon (Introduction of Semiconductor Device), 1st ed., pp. 124-125, Baifukan (Mar. 30, 1995)). The light-emitting part having a double hetero-junction structure is obtained using a junction structure of a light-emitting layer with n-type or p-type cladding layers sandwiching the light-emitting layer. In conventional multi-color light-emitting devices, each of the cladding layers sandwiching the GaXIn1xe2x88x92XN (0xe2x89xa6Xxe2x89xa61) light-emitting layer is generally composed of n-type or p-type aluminum gallium nitride (AlXGa1xe2x88x92XN: 0xe2x89xa6Xxe2x89xa61) (see, (1) JP-A-2001-168384, (2) JP-A-11-289108 and (3) JP-A-10-22525, all cited above).
However, a low-resistance aluminum gallium indium nitride (Alxcex1Gaxcex2Inxcex3N: 0xe2x89xa6xcex1xe2x89xa61, 0xe2x89xa6xcex2xe2x89xa61, 0xe2x89xa6xcex3xe2x89xa61 and xcex1+xcex2+xcex3=1) having a p-type conduction cannot be readily formed due to the non-degenerated valence band structure peculiar to the wurtzite-structure crystal (see, Toshiaki Ikoma and Hideaki Ikoma, Kagobutsu Handotai no Kiso Bussei Nyumon (Guide for Basic Physical Properties of Compound Semiconductor), 1st ed., page 17, Baifukan (Sep. 10, 1991)). According to conventional techniques, in order to obtain a p-type group III nitride semiconductor layer having low resistance, it is necessary to form a group III nitride semiconductor layer by intentionally adding (doping) a p-type impurity such as group II element and then, annealing the layer to eliminate a hydrogen atom (proton) therefrom (see, JP-A-5-183189).
Furthermore, even if the low-resistance Alxcex1Gaxcex2Inxcex3N (0xe2x89xa6xcex1xe2x89xa61, 0xe2x89xa6xcex2xe2x89xa61, 0xe2x89xa6xcex3xe2x89xa61 and xcex1+xcex2+xcex3=1) layer is obtained through cumbersome annealing or the like, when the layer is used as a barrier layer, the order of stacking GaXIn1xe2x88x92XN (0xe2x89xa6Xxe2x89xa61) layers constituting the light-emitting layer is limited. For example, in constructing a light-emitting layer for multi-wavelength light emission by superposing three GaXIn1xe2x88x92XN layers each having different band gaps, namely, different light emission wavelengths, a GaXIn1xe2x88x92XN layer having the smallest band gap of light emission with the longest wavelength must be disposed in the middle of three layers (see, JP-A-2001-168384 supra). This is necessary so that the radiation recombination of a hole and an electron injected to emit light can be generated on average in each layer. More particularly, this is because in group III nitride semiconductors such as Alxcex1Gaxcex2Inxcex3N (0xe2x89xa6xcex1xe2x89xa61, 0xe2x89xa6xcex2xe2x89xa61, 0xe2x89xa6xcex3xe2x89xa61 and xcex1+xcex2+xcex3=1), the diffusion length (this corresponds to mobility) of a hole is about one order of magnitude smaller than that of an electron.
In other words, the problems in conventional techniques, where barrier layers sandwiching a light-emitting layer having a multilayer structure comprising a plurality of GaXIn1xe2x88x92XN (0xe2x89xa6Xxe2x89xa61) layers emitting multi-wavelength light each is constructed from Alxcex1Gaxcex2Inxcex3N (0xe2x89xa6xcex1xe2x89xa61, 0xe2x89xa6xcex2xe2x89xa61, 0xe2x89xa6xcex3xe2x89xa61 and xcex1+xcex2+xcex3=1), are that (1) a low-resistance p-type conductive layer suitable as a barrier layer cannot be readily obtained and (2) due to a large difference in the diffusion length between a hole and an electron, the order of stacking of constituent layers of the light-emitting layer is limited.
The present invention has been made in consideration of the above problems of the prior art. It is therefore an object of the present invention to provide a compound semiconductor light-emitting device having a pn-junction type hetero-junction structure light-emitting part, where (A) a p-type low-resistance layer and a n-type low-resistance layer can both be readily constructed without the need for a cumbersome post-process, and (B) the barrier layer is composed of a compound semiconductor material having no large difference in mobility between holes and electrons, thereby facilitating construction of the light-emitting part.
More specifically, the above object of the present invention has been achieved by providing the following:
(1) a pn-junction type compound semiconductor light-emitting device comprising a substrate composed of a crystal, a first barrier layer provided on the substrate and composed of an undoped boron phosphide-base semiconductor of a first conduction type, and a light-emitting layer of a first or a second conduction type provided on the first barrier layer and comprising a plurality of superposed constituent layers composed of group III nitride semiconductors each having a different band gap, wherein the constituent layer of the light-emitting layer provided closest to the first barrier layer is a first light-emitting constituent layer composed of a group III nitride semiconductor containing phosphorus (P);
(2) the pn-junction type compound semiconductor light-emitting device as described in (1) above, wherein the substrate is a silicon (Si) single crystal substrate;
(3) the pn-junction type compound semiconductor light-emitting device as described in (1) or (2) above, wherein each of the constituent layers of the light-emitting layer is composed of gallium indium phosphide nitride (GaXIn1xe2x88x92XP1xe2x88x92YNY: 0xe2x89xa6Xxe2x89xa61 and 0 less than Y less than 1) or gallium phosphide nitride (GaP1xe2x88x92YNY: 0 less than Y less than 1);
(4) the pn-junction type compound semiconductor light-emitting device as described in any one of (1) to (3) above, wherein the first barrier layer has a band gap larger than that of any of the plurality of light-emitting constituent layers constituting the light-emitting layer by at least 0.1 eV or more;
(5) the pn-junction type compound semiconductor light-emitting device as described in any one of (1) to (4) above, wherein an intermediate layer composed of a group III nitride semiconductor is formed on the surface of the first barrier layer and the first light-emitting constituent layer is joined to the intermediate layer;
(6) the pn-junction type compound semiconductor light-emitting device as described in (5) above, wherein the intermediate layer is composed of a group III nitride semiconductor having a band gap larger than that of the group III nitride semiconductor of the first light-emitting constituent layer;
(7) the pn junction type compound semiconductor light-emitting device as described in (5) or (6) above, wherein the intermediate layer is composed of a group III nitride semiconductor comprising an element constituting the group III nitride semiconductor of the first light-emitting constituent layer;
(8) the pn-junction type compound semiconductor light-emitting device as described in any one of (1) to (7) above, wherein a second barrier layer composed of an undoped boron phosphide-based semiconductor of second conduction type is provided on the surface of the uppermost light-emitting constituent layer of the light-emitting layer;
(9) the pn-junction type compound semiconductor light-emitting device as described in (8) above, wherein the uppermost light-emitting constituent layer of the light-emitting layer is composed of a phosphorus (P)-containing group III nitride semiconductor of first or second conduction type;
(10) a method for producing a pn-junction type compound semiconductor light-emitting device, which comprises forming a first barrier layer composed of an undoped boron phosphide-base semiconductor of first conduction type on a substrate composed of a crystal, and further forming, on the first barrier layer, a light-emitting layer of first or second conduction type and comprising a plurality of superposed constituent layers each composed of group III nitride semiconductors having different band gaps, wherein the constituent layer of the light-emitting layer provided closest to the first barrier layer is a first light-emitting constituent layer composed of a group III nitride semiconductor containing phosphorus (P);
(11) the method for producing a pn-junction type compound semiconductor light-emitting device as described in (10) above, which comprises forming an intermediate layer composed of a group III nitride semiconductor on the surface of the first barrier layer, and joining the first light-emitting constituent layer to the intermediate layer;
(12) the method for producing a pn-junction type compound semiconductor light-emitting device as described in (10) or (11) above, which comprises forming a second barrier layer composed of an undoped boron phosphide-base semiconductor of second conduction type on the surface of uppermost light-emitting constituent layer of the light-emitting layer; and
(13) a white light-emitting diode comprising the pn-junction type compound semiconductor light-emitting device described in any one of (1) to (9) above.